The present invention relates to burners, in particular, line burners for use in web supporting and drying apparatus, although other applications are within the scope of the invention.
According to conventional combustion science, each type of burner flame (e.g., premix flame, diffusion flame, swirl flame, etc.) burns with a different optimal stoichiometric mix of fuel to combustion air, by which low emission concentrations in the burner flue gas appear. It is therefore important to control or maintain the desired optimal stoichiometry of the burner. Failure to closely regulate the burner air/fuel ratio over the range of burner output can lead to poor flame quality and stability (flameout, yellow flames, etc.) or excessive pollution (high NOX, CO).
The turn-down ratio of a burner is the ratio of a maximum firing rate to a minimum firing rate for a particular burner, where firing rate is the measure of the amount of fuel gas consumed per hour, such as BTU/hour. A high turn-down ratio is preferred, since this indicates that the burner is consuming less fuel at the minimum firing rate.
U.S. Pat. No. 5,662,467, the disclosure of which is hereby incorporated by reference, discloses a nozzle mixing line burner having a combustion chamber and a nozzle body having two channels, each of which receives air and fuel. The mixture of air and fuel from each channel is discharged into the combustion chamber where they are mixed. However, at most turned-down firing conditions (i.e., low firing rates), mixture is inadequate and flame quality is diminished.
It would therefore be desirable to improve the flame quality of line burners especially at low firing rates.
The problems of the prior art have been overcome by the present invention, which provides a burner having improved flame quality even at high turn-down ratios or low firing rates. The burner includes a flow director which is preferably a bent sheet or plate positioned in the burner to alter the flow geometry of the air component into a series of channels where the air mixes with the fuel. Preferably the flow director is perforated, the perforations providing a second avenue for the flow of air into the mixing channel.